The Differential is an integral part of all cars. Placed indirectly on the powertrain the main function of the Differential is to let the driven wheel rotate at different RPM. This is highly critical during turning. Consider a pair of wheel negotiating a RIGHT turn. From the below image it can be easily comprehended that the Left wheel has to travel more distance than the right wheel.
If both these wheels are connected using a shaft then the turning cannot be accomplished without the wheels slipping. Let us consider the simplest and widely used configuration of differential called the Open Differential. The power from the engine is transferred to the ring gear through the pinion gear. The ring gear is connected to the Spider gear. The Spider gear is allowed to make two kinds of rotation, first along with the Ring Gear and second along its own access. The spider gear meshes with two Side gears which each drive the left and right axial. Now let us consider different cases.
When the car is moving straight
The power from the pinion is transferred to the Ring Gear. The Spider Gear rotates with the ring gear but doesn’t rotate along its own axis. The spider gear pushed the two sides which rotate at the same speeds. The entire Spider-Side gear assembly will move as a single unit. Thus the two wheels rotate with the same RPM.
When the car is turning
When the car is making a turn, the spider gears play a pivotal role. Apart from rotation with the ring gear, the spider gear also rotates along its axis. When properly mesh both the side and spider gear should have the same peripheral velocity. So when one of the side gear rotates slower it induces a spinning velocity in spider gear which makes it spin on its axis. Since the peripheral velocity of spider and side gears should be same, the spinning velocity of the spider gear makes the opposite side rotate faster. Mathematically the peripheral velocity of the slower side gear is the difference of the rotational velocity and the spinning velocity, while that of the faster side gear is the sum of the rotational velocity and the spinning velocity. Thus the two wheels can rotate in different RPMs and turn without slipping is accomplished.
One of the greatest disadvantages of the Open Differential is that when one wheel is on a surface with good traction and another wheel on a slippery surface with less or no traction, then the differential sends most of the power to the wheel with less or no traction. In the above picture, the front right wheel is stuck in the mud and the front left has almost no traction. The Open differential will send more power to the left wheel rather than the right.
To overcome such disadvantages some SUV manufacturers are moving towards limited slip Differential. One such is the TORSEN differential which is the trade mark of JTEKT Corporation. It has many patented components and is the most ingenious method of providing differential action while overcoming the traction differential problem.
The components of the Torsen differential is quite different from the Open differential. The wheel axial is connected to a worm gear (orange in the above GIF), which in turn is connected to the worm wheel (blue) which is a helical gear. On observing the point of intersection it can be understood that a spinning worm gear can rotate the worm wheel along its axis but the worm wheel will not be able to rotate the worm gear.
A pair of such worm wheel – worm gear is fitted to the case. Each end of the worm wheel is fitted with a helical gear that meshes with the adjacent worm wheel. Now lest consider different cases.
When the car is moving straight
In this condition, the power from the engine is transferred to the differential case through the Ring gear. Since the worm wheel cannot rotate the worm gears it will not spin around its axis. The worm wheel will push and turn the worm gear and hence the whole mechanism moves as a single solid unit.
When the car is turning
During turning either one of the wheels will rotate faster. The faster-moving worm gear will induce a motion on the worm wheel thus making it rotate about its own axis. The rotating worm wheel transfers its motion to the adjacent worm wheel through the spur gear which causes it to rotate in the opposite direction. This causes the opposite worm gear to rotate at a speed relatively slower than that of the differential casing in the opposite direction. The meshing spur gears at the ends of the worm wheel will make sure that the worm wheels are spinning at the same speed thus giving a perfect differential action.
Let now see how the Torsen Differential overcomes the Traction Difference Problem.
Due to the difference of traction at the wheels, one of the worm gear will rotate excessively faster than the other. This causes the corresponding worm wheel to rotate along its own axis. The spur gear makes the adjacent worm wheel rotate in the opposite direction. But as I said earlier the worm wheel cannot rotate the worm gear. This locks the whole mechanism and both the wheel turn together at the same RMP. Equal torques are sent to both the wheel and this helps the vehicle move.
If you are familiar with other limited differential technology you might have noticed that the Torsen has a great advantage compared to the others. The other technologies allow the drive wheel to slip for a limited amount of time before it locks the mechanism, the Torsen is instantaneous. This means as soon as your vehicle encounters a traction difference situation the differential mechanism is locked.
Animation Credits: http://www.learnengineering.com